The present invention relates to methods and apparatus providing a user interface that reduces or eliminates safety concerns with high power imaging, and, in particular, serve to provide mechanical, visual and audible feedback during high MI imaging on a ultrasound system.
Ultrasound imaging involves using a transducer, generally comprising mechanical/electric converters, such as piezoelectric elements, to transmit ultrasound waves into a subject and receive the echoes thereof. During transmit, the piezoelectric elements are excited with an electrical signal to vibrate at a selected frequency thereby generating an ultrasound signal. By selectively exciting the individual elements, the ultrasound signal can be steered and focused. During receive, the piezoelectric elements are excited by the returning echoes and, in turn, output electrical signals that can be processed to create an image of the insides of the subject.
Much of the current innovation in medical ultrasound diagnostic equipment and procedures focuses on harmonic imaging. It has been found that tissue structures within a body produce echoes at a harmonic of the frequency of the impinging signal (xe2x80x9charmonic echoesxe2x80x9d). In harmonic imaging, a signal of a fundamental frequency is transmitted by a transducer into a patient and an image is constructed using the echoes from the patient exhibiting a harmonic frequency of the fundamental frequency. The harmonic echoes, while being more accurate of the structure being imaged, are significantly weaker than echoes exhibiting the fundamental frequency. This presents challenges in designing harmonic imaging systems.
One method of improving harmonic imaging involves the sequential transmission of waveforms with alternating polarities. Upon receive, the echoes resulting from the alternating waveforms are combined so as to eliminate parts of echoes exhibiting the fundamental frequency. Using this methods, it has been claimed, the harmonic content can be increased as much as 6 dB. One example of such a method is described in U.S. Pat. No. 5,833,613 to Averkou et al.
A method to improve harmonic imaging using arbitrary wave functions, is currently under investigation. Standard ultrasound transducers are excited by pulse generator driver circuits which generate rectangular waveforms. When excited in this manner, transducers emit signals, which show up as noise, exhibiting a frequency that is a harmonic of the desired frequency along with the desired signal. The use of a so-called arbitrary wave functions has been explored as a means to lower the amount of noise (and especially the transmitted harmonic noise) output by a transducer. Arbitrary wave function driver circuits output a shaped excitation waveform, typically using Gaussian or Hamming modulation. At least one manufacture claims to suppress the transmitted second harmonic frequencies by 30 dB using arbitrary waveforms. Example of such a method are described in U.S. Pat. No. 5,675,554 to Cole et al. and U.S. Pat. No. 5,740,128 to Hossack et al.
Unfortunately, both of the above-described techniques require complex and expensive hardware to implement. Further, only the first method (the sequential transmission of waveforms with alternating polarities) actually enhances the second harmonic, but only by about 6 dB. The first technique is also subject to motion artifacts. Finally, both methods are limited by the noise floor inherent in any ultrasound system.
The present inventors believe that future improvements in ultrasound imaging, including harmonic imaging, require the use of high pressure transmit waveforms. In general, more contrast is obtained with increased ultrasound pressures. Tissue generates second harmonic pressure proportional to a function of the pressure of the transmitted fundamental frequency, distance, and frequency of the fundamental frequency.
Due to current limitations in transducer design, specifically a lack of bandwidth, most harmonic imaging systems only monitor the second harmonic of the fundamental frequency of the transmit signal. In the future, technologies such as single crystal transducers will provide significant increases in bandwidth, making it possible to monitor the second, third and possibly even the fourth harmonic with a single transducer, assuming the transmit waveform has enough power to generate such higher order harmonics. It is hoped that constructing an image using the higher harmonics will provided even more contrast for even clearer images. Generally speaking the pressure of the nth order harmonic is proportional to P1n. This relationship requires the use of higher fundamental pressures so as to yield higher harmonic pressures.
Unfortunately, current limitations imposed by the U.S. Federal Drug Administration (the FDA) limit the output of medical ultrasound transducers to 1.9 MI.
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The 1.9 MI limitation is based mostly on safety concerns that have not been proven in vivo. Nevertheless, if high MI ultrasound devices are to become reality, the supposed safety concerns will have to be addressed in a positive pro-active manner.
Accordingly, the present Inventors have recognized a need for methods and apparatus for reducing safety concerns with high MI imaging, and, in particular, have realized a new an improved user interface providing feedback and warnings during high MI imaging.
A user interface for providing increased safety when driving a transducer of an ultrasound imaging device at high MIs. An ultrasound imaging device is provided with a first controller for adjusting the output of a transducer and a second controller for high MI imaging. A control circuit, in response to input from the first controller adjusts the output power level of the transducer and when a power level over a predetermined value is requested, requires input from the second controller prior to adjusting the output of the transducer to the requested power level. Alternatively, the first control can be used to set the transducer for low MI imaging, while the second control sets the transducer for high MI imaging. The ultrasound imaging system can also be provided with a variety of display modes to facility safe high MI imaging. For example, a graph of MI vs. depth can be displayed in conjunction with the ultrasound image and/or the portions of the image that receive the highest MI can be highlighted.